What about the inside?

As an adjunct to my thickness sander thread, I thought I'd start a new related thread.

I joined a very rough 1" thick 60+year old redwood board (4" x 36") down to about 3/4 to get two smooth sides. I then resawed the board to split it in half (using my brand new 5/8" Shopsmith band saw blade...cut like butter!). Next I ran the the rough sawn sides on each piece through my new thickness sander (100 grit) to remove the resaw marks (three passes per side did it!). I next split each of these two pieces in half. The insides of these four pieces are too thin to run thru the Grizzly without a backer board (see the Thickness Sander Thread). My question is this: How much dressing of the boards do you do that go on the inside of the dulcimer? I have usually been dressing them down to at least 150 grit. Can they say rough sawn? Obviously where the tops and bottoms are glued to the sides, the surface needs to be smooth to get a good bond. But what about the rest of the insides?